1. Field of the Invention
This invention relates to video signal memories. More particularly, but not exclusively, this invention relates to memories which are suitable for use in a high definition video system, and more particularly still to memories which are suitable for use in special effects equipment for such a video system.
2. Description of the Prior Art
The standard television signal transmitted in the United Kingdom is a PAL signal of a 625-lines per frame, 50-fields per second system, and the PAL, NTSC and SECAM signals transmitted in other countries use similar or slightly lower line frequencies (for example 525 lines per frame), and similar or slightly higher field frequencies (for example 60 fields per second). While there is no immediate prospect of significant changes in these transmitted signals, there is an increasing requirement for higher defintion video systems. Such systems can be used, for example, in film-making, in closed circuit television systems, in satellite communication systems and in studio use generally. One such proposed high definition video system uses 1125 lines per frame and 60 fields per second. This proposed system also uses a 5:3 aspect ratio instead of the 4:3 aspect ratio now usual for television receivers.
The special effects which can be applied to a video signal are well known. Thus, for example, images on a cathode ray tube can be off-set (moved in any direction), scaled (expanded or compressed in size), rolled (rotated in two or three dimensions) and so on.
One way of achieving such special effects, which will be referred to in more detail below, involves converting an input analog video signal into digital form, modifying the individual input digital signals in a field memory, and reading from the field memory to derive the required output digital signals. In the proposed high definition video system referred to above, the input analog video signal is sampled 2048 times per horizontal line scan, so the sample frequency is 69.12 MHz and the sample interval is approximately 14.7 nanoseconds. The time available for writing each digital signal into the field memory is therefore somewhat less than 14.7 nanoseconds because of the modification step in special effects, and the problem is to provide a memory capable of operating at this very high speed.